The lightning spell was the signature attack move by the Archmage.
In 984, the Archmage used the spell on Hudson when he attempted to assassinate Prince Malcolm. When Goliath, Hudson and Demona tracked the Archmage to his lair, the Archmage used the spell again, blinding Hudson’s left eye. ("Long Way to Morning")
The magic of the spell simulated lightning that the Archmage fired through his fingers. How lethal the spell was is unknown, but it was capable of blasting rocks apart.
The direct effect of the spell is to ionize the particles of air between the caster and whatever he wishes to strike; thus, it does not directly affect a living being and does not need to be seen and heard to be effective; Jeffrey Robbins, for example, is as vulnerable to the Lightning spell as any other living being despite his blindness. 
- Fulminous venite
- Come lightning 
When the Archmage says the spell, it sounds as if he is saying "benite" (blessed), instead of "venite" (come). The English captions on the DVD transcribe it as "benite." It also written as "fulminus benite" in Demona's Revenge, a novelization of "Long Way to Morning".